machine-config-operator: Use ostree-container release image for mcbs

[cgwalters]

This changes the CI configuration for the mcbs branch to use
openshift/os#657

[cgwalters]

Based on my reading of https://docs.ci.openshift.org/docs/architecture/ci-operator/#testing-with-an-ephemeral-openshift-release I think this is what we want.

[openshift-ci]

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[openshift-ci]

@cgwalters: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/rehearse/openshift/machine-config-operator/mcbs/e2e-gcp-upgrade	6244a8c	link	unknown	/test pj-rehearse
ci/rehearse/openshift/machine-config-operator/mcbs/e2e-gcp-op	6244a8c	link	unknown	/test pj-rehearse
ci/rehearse/openshift/machine-config-operator/mcbs/e2e-agnostic-upgrade	6244a8c	link	unknown	/test pj-rehearse
ci/rehearse/openshift/machine-config-operator/mcbs/images	6244a8c	link	unknown	/test pj-rehearse
ci/rehearse/openshift/machine-config-operator/mcbs/e2e-aws	6244a8c	link	unknown	/test pj-rehearse
ci/rehearse/openshift/machine-config-operator/mcbs/e2e-gcp-op-single-node	6244a8c	link	unknown	/test pj-rehearse
ci/prow/pj-rehearse	6244a8c	link	false	/test pj-rehearse

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[cgwalters]

Hmm right. It looks like we need to explicitly enable mirroring the image I manually pushed to the coreos namespace into the build clusters via mirroring or so?

Or, I guess I could push these images to quay.io/cgwalters or something to make the "it was built externally" even clearer. Hmm. The nice thing about using the coreos namespace on api.ci is that we have set up team authentication there etc. But, eh.

[cgwalters]

#24223

[cgwalters]

Closing in favor of #24225